Low capacity filament transformer system



July 25, 1944. J. L. FINCH LOW CAPACITY FILAMENT TRANSFORMER SYSTEMOriginal Filed March 15, 1940 2 Sheets-Sheet l INVENTOR Jarrzeo LlihclvBY wvt ATTORNEY July 25, 1944. J F H 2,354,585

Low CAPACITY FILAMENT TRANSFORMER SYSTEM Original Filed March 15, 1940 2Sheets-Sheet 2 INVENTO R' WL.M-

ATTORN EY Patented July 25, 1944 .UNl'lED STATES PATENT. OFFICE LOWCAPACITY FILAMENT TRANS- FORMER SYSTEM James Leslie Finch, am Rockaway,is. Y., assignor to Radio Corporation of America, a corporation ofDelaware '3 Claims. ('01. 175-356) ferring to the accompanying drawings,in which:

This invention relates to a new anduseful transformer device which isparticularly adapted to short wave radio frequency generators andamplifiers which employ thermionic devices and particularly to suchdevices wherein the cathodes are heated by alternating current andwherein the anodes are maintained at an --appreciably generated oramplified wave voltage.

This application is a division of my copending application Serial No.324,035, filed March 15, 1940, Patent No. 2,314,083 granted March 16,

1943, the claims in this application beingdirected to the transformerconstruction.

A system according to this invention will be found particularlyadvantageous when anodes of the thermionic devices are fitted with bulkymetallic fin devices, over which air is blown for the purpose of coolingthem. In such devices, the anodes of the thermionic devices can be grunded with respect to the electric waves which are being generated oramplified.

1n the prior art, it has been the usual practice to ground the cathodeand to subject the anodes to the electric wave voltages. The highcapacity of the anodes to ground and to other objects when bulkymetallic cooling found objectionable. The resultant high capacity maylimit the maximum usable frequency to a materially lower value thanwould be otherwise possible. Further, when the anode was grounded andaccordingly both the grid and the cathode of the thermionic devicessubjected to high voltages, it was necessary to supply heating currentto the cathode through chokes or' tuning coils. Such a method proved tobe a handicap in many cases. One detrimental effect was that it made itmore diflicult to change the frequency of the tuned circuits involved.Another detrimental factor was that the chokes, etc., introduced lossesboth in the high frequency wave voltage circuit and in the heatercircuit. This method of supplying cathode heater power hasiinsareusedis' generally been found impractical at radio fr'equencywaves below twenty megacycles.

It is an object of my invention to improve such a system by employing atransformer inwhich the windings are isolated from ground and by theintroduction of an improved radio frequency circuit which employs tuningmeans between the grid and the anode of the thermionic device for .thepurpose of preventing regeneration due to grid-anode capacity and forthe purpose of preventing excitation power from feeding through to theoutput due to the last mentioned capacity.

Fig. 1 is a schematic circuit arrangement of the system of thisinvention;

Fig. 2 is a section of the improved filament transformer;

Fig. 3 is a plan view of Fig. 2: and

Fig. 4 is a modified sectional view similar to that of Fig. 2, exceptthat both the primary and secondary are insulated from the core by anair gap.

Referring now in detail to the drawings-Fig. 1 shows a circuit diagramof a radio frequency amplifier having a range up to twenty megaoycles infrequency. The thermionic triode device is of a type similar to an RCA893 tube and is indicated as i having an anode 2, grid I and filament 4.The filament is shown as consisting of three separate strands connectedtogether at their lower ends. The RCA 893 thermionic triode actually hassix such strands connected together at their lower ends, so that in thefigure each line is representative of two strands in parallel. Ametallic fin assembly, preferably of copper, surrounds the anode 2 forcooling the same. This cooling assembly consists of a hub 0 of solidcopper into which the anode is soldered. A-plurality of fins I extendradially out from hub 0. A blast of air is blown between the ilns, asindicated by the arrows. The radio frequency circuit employs a tuningmeans between the grid and anode for the purpose of preventingregeneration due to the grid-anode capacity and for the purpose ofpreventing excitation power from feeding through to the output'due tothe grid-anode capacity. This tuningmeans consists essentially of andinductance coil 0 and a trimming condensed 9, which also includes biasbypass condenser II and a plate bypass condenser ll having a commonground connection between the two. The input circuit is disposed betweenthe grid and filament of the thermionic tube. This circuit is tuned bymaking the reactance of coil II equal to the combined reactance of thegrid-to-filament capacity, and the tuning condenser II. The source ofexcitation-power is represented by a generator l4. Generator l4 suppliescurrent to coil ll which is inductively coupled to coil l2 and I havefound it advisable to shield against capacitive coupling between thesetwo circuits by This invention will best be understood by remade equalto the reactance of the filament-toground capacity, thefllament-to-anode capacity, and the capacity of tuning condenser ll. Theoutput circuit is coupled in some suitable manner to the load. In thiscase, this tcoupling is obtained by means of output coil is, which isinductively coupled to tuning coil l1. Thespecial low capacity filamenttransformer is indicated at and comprises an iron core assembly, primarycoils 22, 23 and 24, and secondary coils 25, 26 and 21. The descriptionof this transformer will be more fully described with reference to Figs.2 and 3. The plate voltage source is indicated as +3 and is connected tothe anode as sembly. Thus, the anode is subjected to the full voltage ofthe plate supply. However, it is subjected to negligible radio frequencyvoltage, since any such voltage is bypassed to ground through condenseri I. The anode current flows to the illament, through the individualtransformer windings and through tuning coil I], thus returning to thenegative side of the anode supply through the common ground connection.Bias voltage for the amplifier system is introduced from the sourceindicated as C through grid-anode tuning inductance I to the grid 3. Thereturn circuit for the grid current is through the filaments, throughtuning coil l1 and the common ground connection to the positive end ofthe grid bias source. The radio frequency connections are made to thefilament by means of bypass condensers 28, shown located adjacent to thelow capacity transformer assembly 20.

Referring now in detail to Figs. 2 and 3, it will be noted that thisconstruction differs from the ordinary filament transformer, as known inthe prior art, in that the secondary windings are isolated from the ironcore and from the primary windings by an air gap, that is, there isprovided a space between the core and the coil winding in which there isno metallic substance or solid dielectric material. Such a constructioninsures a low capacity between the secondary windings and other adjacentobjects. The secondary windings 25, 2| and 21 are shown supported onangles 28 and 30 which in turn are supported by four insulators 3i, lid,32 and 32a. The iron core 2| is indicated as being substantiallycircular in section. This allows the minimum spacing for a givenimpressed voltage. The circular core is constructed by having thecentral laminations larger than the outermost laminations, that is,

each lamination on each side of the center line decreases proportionallyin size on each side of the central line, thus providing a coreessentially circular in cross-section. The secondary windings arearmored with a non-magnetic metal 35 which protects the insulation ofthe individual conductor turns from the radio frequency field. It alsoprotects the insulation from damage in case of an are between thesecondary windings and the core. This armor 3! is connected to thecommon point of the three secondary windings as indicated in the circuitdiagram of Fig. 1.

Located directly below each individual secondary winding is a primarywinding 22, 23 and 24. Core 2| is mechanically secured to a base plate36 by means of a plurality of strap angles 31 secured to plate I bymeans of screws 2!. Bypass condensers 28 are mounted on angle 2!adjacent to each secondary winding and connected thereto by leads 3! andll.

Fig. 4 shows a further modification from that of Figs. 2 and 3 in whichthe core is isolated by an air gap from the primary in the same manneras shown and previously described in which the secondary was isolated.The core in this case will also be supported by a strap II secured toinsulators 3| and 82. This construction will further decrease thecapacity.

While I have described my invention as related to a system usingthree-phase filaments and have shown the same as connected withthreephase transformers, this invention applies equally well to singlephase filament supply and three single phase transformers can be usedfor a three-phase supply. Also, the transformers can be arranged in anyother of the forms known in the prior art.

What is claimed is:

1. A low capacity transformer especially adapted for use with thefilament system of a vacuum tube to be used in high frequency circuitscomprising a closed laminated core, a primary winding wound on saidcore, and a secondary winding through which said core passes, thesecondary being so spaced from said core and primary that there isrelatively small capacity between said secondary and said core andprimary, said primary and secondary being ar ranged in the same planeand having spaced substantially parallel axes, said primary andsecondary moreover being arranged to physically encircle differentportions of said core.

2. Apparatus as claimed in the preceding claim characterized by the factthat said secondary winding is shielded by being enclosed within anon-magnetic metallic casing.

3. A transformer especially adapted for the illament circuit of a vacuumtube to be used in high frequency circuits comprising a laminatedmetallic core substantially circular in cross-section, a primary windinglinked with and spaced from said core and a secondary winding linkedwith and spaced from said core, said secondary winding being encasedwithin a metallic shield, the spacings of said windings. with respect tothe core and with respect to each other being such that relatively lowcapacity exists betwen the secondary and both the core and primarywinding, said windings moreover being arranged in substantially the sameplane and about spaced substantially parallel axes, said windings beingarranged furthermore to encircle different portions of said core.

JAMES LESLIE FINCH.

